(1) Field of the Invention
This invention relates to a method of manufacturing a cylindrical non-aqueous electrolyte secondary cell, and more particularly to a method of manufacturing a cylindrical non-aqueous electrolyte secondary cell usable for a battery-powered vehicle and the like, wherein a high power density is required.
(2) Description of the Prior Art
A conventional cell of this type generally has a spiral type electrode assembly in which a sheet type positive electrode and negative electrode are spirally wound with a separator disposed therebetween. Electric current is collected from the spiral electrode assembly through conductive labs that are attached on each end part of the aforementioned electrode and current terminals that are electrically connected to the conductive tabs. In a cell of this type, when the cell is a small-sized cylindrical non-aqueous electrolyte secondary cell with a small electric current, the current collecting effect can be sufficient. However, when the cell is a large-sized one with a large electric current, it is difficult to obtain a satisfactory current collecting effect.
In view of the above problem, Japanese Unexamined Patent Publication No. 6-267528 discloses a cylindrical non-aqueous electrolyte secondary cell having the following features. The cell has a sheet type current collector in both the positive electrode and the negative electrode. In both electrodes, a lengthwise end area of the current collector is protruded from a separator, and at the same time the lengthwise end area to which a lead is to be attached is exposed without an active material applied thereon. Then, a positive electrode lead-attaching area and a positive electrode lead, and a negative electrode lead-attaching area and a negative electrode lead are respectively welded.
However, such cylindrical non-aqueous electrolyte secondary cells have certain drawbacks. For the cells of such construction, spot welding is generally employed to weld a lead to a lead-attaching area in each electrode. In this case, when the current collectors have a small thickness of less than 50 .mu.m, the lead-attaching area thereof is apt to be damaged since, in spot welding, pressure is applied to the area. To prevent the damage, the lead-attaching area requires a reinforcement means, for example, by metal foil and the like, which consequently lowers production efficiency.
As a solution to this problem, laser welding can be employed to weld a lead to a lead-attaching area in each electrode. Laser welding is a non-contact type welding method, in which no pressure is applied on to lead-attaching areas. The reinforcement means to the lead-attaching areas can therefore be eliminated, and workability in manufacturing a cell is thereby improved.
However, laser welding incurs a different problem.
In the present state of the art, in welding a lead to a lead-attaching area, it is extremely difficult to direct a laser beam to an exact position to be welded. Therefore, when a laser beam is applied onto a lead, an area in the electrode assembly except the region to be welded is also exposed to the laser beam indiscriminately. As a result, the irradiated area of the lead is fused and holes are formed, and the laser beam passes through the holes and reaches the internal electrode assembly, resulting in fusion of the electrode assembly and thereby a short circuit.